1. Field of the Invention
This invention relates to a stimulable phosphor sheet transfer apparatus for transferring a stimulable phosphor sheet in a radiation image recording/reproducing system in which the stimulable phosphor sheet is exposed to radiation through an object and is scanned with a stimulating ray to emit light in the pattern of the stored energy of radiation, and more particularly to a stimulable phosphor sheet transfer apparatus for transferring a stimulable phosphor sheet in such a radiation image recording/reproducing system during radiation image information reading operation in which the light emitted from the stimulable phosphor sheet upon exposure to the stimulating ray is detected to obtain an image signal.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to a stimulating ray such as a visible ray, light is emitted from the phosphor in the pattern of the stored energy of the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor and a sheet material bearing thereon a stimulable phosphor layer is referred to as a "stimulable phosphor sheet" and can be used as a recording medium for recording thereon a radiation image information.
There has been proposed a radiation image information recording/reproducing system in which a radiation image of an object is once recorded on a stimulable phosphor sheet as a pattern of stored energy of radiation, the stimulable phosphor sheet is scanned with a stimulating ray, and light emitted from the stimulable phosphor sheet is photoelectrically detected to obtain an image signal which is subsequently processed to reproduce a visible radiation image of the object. For example, see Japanese Unexamined Patent Publication No. 56(1981)-11395, U.S. Pat. Nos. 4,258,264, 4,315,318, 4,387,428 and 4,276,473. The image signal can be reproduced as a hard copy or a visible image on a cathode ray tube, for instance.
FIG. 1 shows an example of a radiation image information reading device for use in such radiation image recording/reproducing systems.
In FIG. 1, a laser beam 1a (as the stimulating ray) of a predetermined intensity emitted from a laser source 1 is caused to impinge upon a galvanometer mirror 2. The laser beam 1a is deflected, as indicated at 1b, by the galvanometer mirror 2 to sweep a stimulable phosphor sheet 3 below the galvanometer mirror 2 in the direction of arrow A (main scanning) while the stimulable phosphor sheet 3 is transferred by an endless belt 9 in the direction of arrow B (sub-scanning) perpendicular to the direction of the main scanning. That is, the stimulable phosphor sheet 3 is two-dimensionally scanned with the deflected laser beam 1b. The parts of the stimulable phosphor sheet 3 exposed to the stimulating laser beam 1b emit light according to the energy of radiation stored therein. The light emitted from the stimulable phosphor sheet 3 upon exposure to the stimulating laser beam 1b enters a light guide member 4 having a light incident face 4a disposed near the stimulable phosphor sheet 3 to extend in parallel to the main scanning line. The light guide member 4 is flat in shape at the front portion 4b thereof and substantially cylindrical at the rear portion 4c thereof. To the rear portion 4c of the light guide member 4 is connected a photomultiplier 5 and the light entering the light guide member 4 from the light incident face 4a impinges upon the photomultiplier 5. The photomultiplier 5 converts the light into an electric signal and delivers it to an image signal information processing circuit 6. The image signal thus obtained is reproduced as a visible image on a CRT 7 or stored in a magnetic recording tape in this particular example.
If an end portion of the stimulable phosphor sheet 3 curls upward away from the surface of the endless belt 9 or an intermediate portion of the stimulable phosphor sheet 3 is raised upward from the surface of the endless belt 9 while the stimulable phosphor sheet 3 is transferred to be two-dimensionally scanned with the stimulating laser beam 1b in order to read the radiation image information stored in the stimulable phosphor sheet 3, the image to be read can be distorted and the stimulable phosphor sheet 3 can be interfered with the light guide member 4. In order to keep flat the stimulable phosphor sheet 3 during transfer for reading the radiation image information, suction force is applied to the stimulable phosphor sheet 3 through the endless belt 9. That is, as shown in FIG. 2, a large number of small holes are provided in the endless belt 9 and a negative pressure generated in first and second suction boxes 10a and 10b below the upper run of the endless belt 9 is applied to the stimulable phosphor sheet 3 through the holes to attract the stimulable phosphor sheet 3 against the surface of the endless belt 9. If only one suction box is provided below the upper run of the endless belt 9, the negative pressure applied to the stimulable phosphor sheet 3 will fluctuate until the stimulable phosphor sheet 3 comes to entirely cover the suction box. This is the reason why a pair of suction boxes 10a and 10b are provided. That is, the first and second suction boxes 10a and 10b bound with each other along the scanning line of the stimulating laser beam 1b. No negative pressure is generated until the leading edge of the stimulable phosphor sheet 3 reaches the downstream edge of the first suction box 10a, i.e., until the first suction box 10a is entirely covered with the stimulable phosphor sheet 3. Thereafter, when the second suction box 10b is entirely covered with the stimulable phosphor sheet 3, the negative pressure in the first suction box 10a is removed and a negative pressure is generated in the second suction box 10b. The negative pressure in the second suction box 10b is maintained until scanning of the stimulable phosphor sheet 3 with the stimulating laser beam 1b is completed. Thus, the stimulable phosphor sheet 3 is kept in close contact with the surface of the endless belt 9 by a negative pressure applied by one of the suction boxes 10a and 10b from the beginning to the end of scanning of the stimulable phosphor sheet 3.
As shown in FIG. 3A, at the time tl when the second suction box 10b - is entirely covered with the stimulable phosphor sheet 3, the negative pressure in the first suction box 10a is abruptly lowered from a predetermined value Pl to zero and at the same time, a negative pressure of the predetermined value Pl is abruptly generated in the second suction box 10b. This is to reduce fluctuation in load on the transfer means. That is, when the stimulable phosphor sheet 3 is attracted against the endless belt 9 under the force of negative pressure, load is exerted on the transfer means including the endless belt due to resistance produced between the endless belt and the suction box in operation, and accordingly the load on the transfer means fluctuates with the number of suction box in operation.
However, when the pressures in the first and second suction boxes 10a and 10b are abruptly changed in the manner described above, transfer speed of the stimulable phosphor sheet 3 changes to cause density fluctuation in a reproduced image in the direction of the sub-scanning.